Since the 1970's, wireless communication systems have seen increased popularity and are now commonplace in both commercial and personal aspects. For example, satellite links, cellular and mobile telephone systems (operating in the 846-891 MHZ frequency range), and more recently, Personal Communication Systems (operating in the 1.91-1.99 GHz frequency range), and many other useful, convenient, and low cost devices have been developed.
One such type of cellular communications system employs a "Personal Station" which is a telephone apparatus for use in a moving vehicle, e.g., an automobile. FIG. 1 illustrates a combination of a "Personal Station"/RF Booster system 10 with the "Personal Station" 12 comprising a vehicular cellular telephone, that may include, for example, a handset (not shown) capable of being placed in a holder of a vehicle's "Hands-Free" Personal Communications Kit to enable a driver (or passenger) to utilize full cellular phone services, i.e., receive and transmit, without the need to hold the phone while the vehicle is in motion. Typically, as shown in FIG. 2, the front end of the receiver device 75 of the Personal Station, is most likely provided with a fixed or variable gain low noise amplifier 65, with sufficient power handling capability to warrant Industry Standards classification of Personal Stations as Class II devices. As all Personal Stations adhere to this Industry standard, each receiver will include such a low noise amplifier. Typically, the gain of such an amplifier ranges from 13-18 dB, and, as shown in the example front end receiver 75 in FIG. 2, may be switched in and out by the provision of switch 72 connected in parallel with the low noise amplifier 65. For instance, when the received input signals are small, the front end receiver 75 switches the low noise amplifier 65 on, for example, by logic control signal 67 which controls the opening of switch 72. Logic control signal 67 is part of a sensor feedback system (not shown) that senses the amplitude of received input signals. The gain of the front end receiver 75 with the low noise amplifier 65 switched on, in this case may be, e.g., 13.5 dB. As a consequence, any noise present at the input is suppressed, and consequently, sensitivity is improved. When the value of the input signal is high enough to overload the mixer, the low noise amplifier 65 is switched off by logic control signal 67 which controls the closing of switch 72. At the time the low noise amplifier is switched out, the gain of the front end becomes -6.5 dB and front end overloading is prevented. The sensitivity in this case is not impaired since the value of signals is high enough to be reliably distinguished from the noise level.
It should be understood that the types of signals contemplated to be processed in such wireless systems include TDMA (time division multiple access) signals and CDMA (code division multiple access) signals. The sensitivity of the Personal Stations receiving these signals is about -110 dBm for TDMA and -104 dBm for CDMA.
As shown in FIG. 1, the Personal Station 12 is coupled to the RF booster 25 via an antenna coupler 15 and coaxial connection cable 20. The RF booster 25, in turn, is connected by a short coaxial cable 48 to the roof antenna 50 of a vehicle such as an automobile. The RF booster 25 typically comprises a transmit duplexer 30a and 30b for coupling signals from the output of Personal Station 12 to a transmitter power amplifier 45 for outputting high power signals through the vehicle's roof antenna 50. On the receiving side, the duplexer 30a and 30b couples input RF signals received from the vehicle antenna to a low noise amplifier 40 and, in turn, to the receiver end of the Personal Station 12. Typically, the RF Booster's low noise amplifier 40 is also a fixed gain device of constant linearity and amplification in the personal station is usually done by increasing the transmitted power of the combination "Booster--Personal Station" as compared to the transmitted power of the Personal Station itself.
Normally, the goal of the RF booster's design is to enhance operation of the Personal Station, typically a Class II unit, into a higher class, e.g., a Class I combination. It is a straightforward task to convert the combination "Booster--Personal Station" to the Class I unit on the transmission side, e.g., by increasing the transmitted power of the RF booster's power amplifier 45. However, it is not so easy to meet Class I requirements of this combination on the receiving side because of the difficulty in meeting sensitivity and linearity requirements simultaneously.
One solution would be to simply provide the low noise amplifier such as contained in the typical receiver of a personal station, to the RF Booster's front end. However, this solution will not meet the sensitivity requirements for Class I Personal Stations.
As it is recognized that an RF booster includes a combination of lossy and noisy connecting cables, e.g., coax cables 20 and 48 as shown in FIG. 1, a Low Noise Amplifier may be provided in the booster to compensate for the cable loss. However, it is the case that if an overall gain of the RF booster is 0 dB, the cables generate enough noise for overall sensitivity to fail the specifications. A traditional method to improve the sensitivity is to increase the gain in the front end. However, if the booster's gain is increased even for the smallest input signals, the power of interfering signals, will be amplified as well resulting in overload of the receiver's low noise amplifier.
It would thus be highly desirable to provide a variable gain and linearity low noise amplifier device for use in wireless communication systems, such as the Personal Station.
Moreover, it would be highly desirable to provide a variable gain and linearity low noise amplifier that would enable receipt of signals from more remote geographical locations, i.e., thus increasing effective range of personal and mobile cellular communication systems with improved sound quality.